绿洲是浩瀚沙漠中的片片沃土，它就像是沙漠瀚海上美丽的珍珠，镶嵌在沙漠里，闪烁着神奇的色彩。那么绿洲是怎样形成的呢？就让下面这篇文章来告诉你答案吧。
An oasis is a fertile spot in the middle of a desert, an island of life in an ocean of temperature extremes. Any oasis always contains one or more springs (a place where water naturally flows out from the ground). Oases make it possible to survive long treks through the desert. In large deserts such as the Sahara, towns cluster around sources of water such as oases and rivers.
What causes an oasis? An oasis is actually a spot in the desert where the elevation is low enough that the water table is right underneath the surface, resulting in the presence of springs. Even in a desert, it rains occasionally, and this produces a water table just above the bedrock, usually several hundred feet below the surface. Sand is veryporous, so most water runs right through it and down to the bedrock.
Deserts consist of many millions of tons of sand. There is only one natural force capable of moving it in appreciable amounts: the wind. Although, in an average dust storm, ten cubic feet (3.05 cubic meters) of air only holds about an ounce of sand, a cubic mile (1.6 cubic km) of air can move about 4,600 tons of it, leading to appreciable erosion. A severe storm is capable of moving as much as 100 million tons of sand and dust.
In certain areas where large quantities of sand are moved by storms, erosion burrows all the way down to the water table, putting it just beneath the surface. Seeds planted in the ground there are capable of sprouting and extending roots into the moist land, producing an oasis.
Sometimes, the oasis produced by the wind can be very large when vast tracts of desert are wiped clean by storms. The great Kharga oasis in the Sahara, for example, is over 100 miles (161 km) long and 12 to 50 miles (19.3 to 80.5 km) in width. The oasis was produced when erosion caused the margins of a great depression to sink down to the water table.
porous: describes something that has many small holes, so liquid or air can pass through, especially slowly 多孔的
（来自：www.wisegeek.com 首都师范大学通讯员刘然供稿 英语点津 Annabel 编辑）
